Muscle contraction in the human body is caused by electrical biosignals. Heart muscle contractions are caused by a biosignal referred to as the electrocardiogram (ECG) signal. The QRS complex describes a region of particular activity in the ECG signal during each heartbeat. Heretofore, various devices have been designed to determine an individual's heart rate by detecting the R-wave portion of the QRS complex.
These prior devices use essentially analog systems and may be described with reference to FIG. 1. Referring to FIG. 1, a conventional heart rate monitor 10 includes a sensor 12, that senses the QRS complex and accompanying noise, and a filter 14 that produces a pulse each time the QRS signal is detected. The pulse from filter 14 is amplified by amplifier 16 and compared with reference 18 by comparator 20. Whenever comparator 20 detects the proper condition, comparator 20 activates a monostable multivibrator or "one-shot" 22. Digital logic circuitry 24 produces a heart rate number to be displayed on LCD display 26 by counting the number of times one-shot 22 is activated. The combination of filter 14, amplifier 16, reference 18, comparator 20 and one-shot 22 is a analog signal processor.
Sensor 12, in the various prior devices, has been an electrode or a microphonic, piezoelectric, photo-optical or capacitive apparatus. Filter 14 has included a filter or a series of filters. Reference 18 has been a fixed voltage reference or inverse exponential voltage developed across a discharging capacitor. Comparator 20 has been a standard comparator or Schmitt trigger threshold circuit.
These primarily analog devices fail whenever the signal to noise (S/N) ratio is low. The prior systems are inaccurate because in a noisy environment they fail to detect certain signal pulses that represent heart beats, and they falsely detect noise peaks as heart beats. The weaknesses of prior devices are discussed in Leger, L., and Thivierge, M., "Heart Rate Monitors: Validity, Stability, and Functionality", The Physician and Sports Medicine, Vol. 16, No. 5, May 1988; and Allen, Douglas, "Heart-rate monitors: The ideal exercise speedometer", Fitness Management, page 34-37, Nov/Dec 1988. A low S/N ratio may be caused by at least three factors: (1) a low amplitude biosignal of the user because of skin impedance which varies from person to person when the person's skin is not properly prepared, (2) vigorous exercise by the user, and (3) poor contact between the sensor and the user.
Medical grade heart rate monitors, such as those found in hospitals, which use chest electrodes mounted onto the user's chest, and an electrode gel interface, do not ordinarily provide spurious results caused by low S/N ratio problems. In these medical grade heart rate monitors, the signal level is high because of the electrode gel and mounted electrodes, and the noise level is low because the electrodes are mounted close to the user's heart and, often, the user is not exercising while his heart rate is being measured.
Wrist worn and hand held heart rate monitors, however, are susceptible to a low S/N ratio because the inherently poor sensor contact with the user's skin provides a low signal level. Moreover, the noise level is high because the sensor is located remotely from the heart and the user is typically exercising.